1. Field of the Invention
The invention pertains to vehicle safety apparatus and more particularly concerns means for utilizing short duration signals to determine the range between a protected vehicle and the forward object which may be a preceding vehicle.
2. Description of the Prior Art
In mass transit systems, a need exists for automatically determining the distance between one vehicle and another. One early signal system, that is still widely employed, is the fixed block system, in which the right-of-way is divided into a series of blocks. The presence of a vehicle in the block creates a response that causes a stop signal to appear at a selected number of blocks to the rear of the vehicle, and a warning signal to appear at a selected number of blocks to the rear of the stop signal. Block sizes and block intervals between signals, for safe operation of the transit system, are functions of the maximum speed capability of the vehicles employed therein, increasing as this speed capability increases. These systems are inflexible, are limited in vehicle capacity and require excessive capital expenditures when utilized in high speed mass transit systems.
Moving block systems have been developed to minimize cost, increase system flexibility, and increase system vehicle capacity. In these systems, relative distance and speed between vehicles are determined with electromagnetic equipment located at a forward position in each vehicle. Transmitted electromagnetic signals propagate along a transmission line, which may comprise the tracks of the mass transit system, until reflected from an obstacle along the right-of-way. The reflected signal is processed to determine the relative distance and speed between the obstacle and the vehicle. These systems employ conventional means for measuring distance electromagnetically and are therefore limited with respect to the minimum distance that may be determined. This minimum distance limitation prescribes the maximum speed of a vehicle in a transit system and also restricts vehicle utilization of the right-of-way.
It is an object of the present invention to provide an electromagnetic distance measuring system, which may be utilized in mass transit systems, that is capable of measuring the relative distance and speed between a vehicle and an obstacle accurately, flexibly, and inexpensively.